refactor(frontend): the last pay-down, part 1 — three dead views deleted, editor rules shared, .page-container + .fs-input canon, rules-shared.css; a one-declaration CSS body is not a shape (#2903, milestone 299 step 5)

The derive queue said the biggest duplicate families were whole views:
TaskViewerView, UserManagementView and LogsView were imported nowhere — left
behind when tasks moved to the editor and users/logs became SettingsView
tabs. Deleted (rule 22). Note/TaskEditorView carried six identical scoped
rules -> editor-shared.css (the .tag-suggest-row gap the scoped copies
actually rendered wins). Three views wrapped the page under three names ->
.page-container in components.css. Three scoped input recipes -> the design
system fs-input recipe (snippet #2336), verbatim, in components.css; width/
box-sizing stay with the caller. The three rules panes share .pane and the
pane heading via rules-shared.css (the auth-shared pattern, #2852).

Ledger: a single-declaration CSS rule keeps its selector in its fingerprint,
so `color: var(--fs-text-tertiary)` under five different names is no longer
a five-file "identical body" family — the first pay-down found that most of
the 148 dup families were exactly this, and nobody would consolidate them.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/assets/editor-shared.css

@@ -78,7 +78,7 @@
   display: flex;
   flex-wrap: wrap;
   align-items: center;
-  gap: 0.4rem;
+  gap: 0.3rem;
 }
 .tag-pill {
   display: inline-flex;
@@ -508,3 +508,36 @@
   opacity: var(--fs-disabled-opacity);
   cursor: not-allowed;
 }
+
+/* Shared by NoteEditorView and TaskEditorView — both carried identical scoped
+   copies of these until #2903 (milestone 299); one source here. */
+.body-tabs-row {
+  display: flex;
+  flex-direction: row;
+  align-items: center;
+  gap: 0.75rem;
+  flex-wrap: wrap;
+  padding-bottom: 0.5rem;
+  border-bottom: 1px solid var(--fs-border-color);
+}
+.body-editor-wrap {
+  min-height: 200px;
+}
+.stream-preview {
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-sm);
+  padding: 0.75rem;
+  background: var(--fs-surface-raised);
+  min-height: 200px;
+}
+.main-diff {
+  flex: 1;
+  min-height: 0;
+}
+.assist-section-title {
+  font-size: 0.78rem;
+  font-weight: 500;
+  color: var(--fs-text-secondary);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}